Switch default SDK to 1.5.0.

Makefile

@@ -10,7 +10,7 @@ TOOLCHAIN = $(TOP)/xtensa-lx106-elf
 
 # Vendor SDK version to install, see VENDOR_SDK_ZIP_* vars below
 # for supported versions.
-VENDOR_SDK = 1.4.0
+VENDOR_SDK = 1.5.0
 
 .PHONY: crosstool-NG toolchain libhal libcirom sdk